As conventional electronic blood pressure meters, there are known those using a cuff wrapped around an upper arm of the subject person for measurement. According to such an electronic blood pressure meter, the air pressure in the cuff (simply referred to as cuff pressure hereinafter) is changed at a predetermined rate, and cardiovascular information obtained during this process (pulse wave, Korotkoff sound, etc.) is used to determine the systolic and diastolic (or maximum and minimum) blood pressure values.
The electronic blood pressure meters based on this principle may be classified into those based on the oscillometric method utilizing pulse wave as the cardiovascular information and the Korotkoff method using the Korotkoff sound as the cardiovascular information. An electronic blood pressure meter based on the oscillometric method detects pulse wave during the process of gradual venting the cuff, and determines diastolic and systolic blood pressure values (which are simply referred to as blood pressure values hereinafter) by evaluating the changes in the amplitude of the pulse wave. On the other hand, an electronic blood pressure meter based on the Korotkoff method detects the Korotkoff sound with a microphone during the process of gradual venting to determine blood pressure values.
In either case, it is necessary to increase the air pressure in the cuff (cuff pressure) above the systolic pressure before measurement can be started, and it is in principle not possible to determine blood pressure values if pressurization is not sufficient or the set pressure is lower than the systolic blood pressure.
According to an electronic blood pressure meter based on the oscillometric method, insufficient pressurization cannot be discovered until a late stage of measurement is reached. Therefore, measurement is continued for some time even when pressurization is insufficient only to find out that the pressurization was insufficient after elapsing a certain time period, and measurement has to be started all over again.
On the other hand, according to an electronic blood pressure meter based on the Korotkoff method, it is possible to discover insufficient pressurization by presence or absence of the Korotkoff sound immediately after starting measurement. However, it cannot be found how insufficient pressurization was, and repressurization by a certain amount may turn out to be insufficient again or an excessive pressurization may take place. Furthermore, because of the use of a highly sensitive microphone, unnecessary repressurization may be made by mistaking surrounding noises for the Korotkoff sound, and unnecessary repressurization may be carried out therefor.
Thus, the conventional electronic blood pressure meter involves the inconvenience that the set value of pressurization must be carefully determined so as to avoid insufficient pressurization, and may cause undue pain and obstruction of blood flow to the subject person by failure to effect a successful completion of a measurement process due to insufficient pressurization. Further, the need for repeated measurement reduces the efficiency of the measurement process.
Generally speaking, a gravely ill patient or a patient during operation requires not only a quick measurement of his blood pressure but also a blood pressure measurement method which is capable of detecting rapid changes in his blood pressure. To meet this need, the method called as the direct method is generally employed for such a purpose. According to this method, a needle is inserted into an artery of the patient, and the internal pressure of the artery is directly measured with a pressure sensor connected thereto and continually displayed. However, this method is not only inconvenient to carry out but also presents the risks of infection and inflicting injury to the patient.
Therefore, lately, there has been introduced the continuous blood pressure monitoring device which carries out the aforementioned indirect measuring process in an intermittent and automatic fashion, the indirect measuring process consisting of the steps of placing a cuff on an upper arm of the patient, increasing and reducing the air pressure in the cuff (which may be referred to as cuff pressure hereinafter) at a fixed rate, and determining blood pressure values according to cardiovascular information such as the Korotkoff sound and the pulse wave obtained during this process. This continuous blood pressure monitoring device is safer and simpler than those using the direct method since all that is necessary is to place a cuff on the subject person.
However, according to such a continuous blood pressure monitoring device, since it is necessary to change the cuff pressure over a range covering the systolic blood pressure value and the diastolic blood pressure value, each measurement process takes from several seconds to more than one minute, and it is difficult to detect rapid changes in the blood pressure.
Further, according to such a continuous blood pressure monitoring device, since the blood flow in the artery must be temporarily obstructed by the cuff, if the measurement is repeated without interruption, the resulting blockage of blood flow prevents accurate measurement, and an undesirable effect may be caused to a patient having a problem with his blood circulation.
Further, if the blood pressure measurement takes place during the process of reducing the cuff pressure, since it is necessary to increase the cuff pressure above the systolic blood pressure before reducing the cuff pressure, any insufficiency of initial pressurization prevents accurate blood pressure measurement, and repeated measurement becomes necessary. For such reasons, the direct method is still widely used for continuous monitoring of blood pressure.